In the use of radiotherapy to treat cancer and other ailments, a powerful beam of the appropriate radiation is directed at the area of the patient that is affected. This beam is apt to kill living cells in its path, hence its use against cancerous cells, and therefore it is highly desirable to ensure that the beam is correctly aimed. Failure to do so may result in the unnecessary destruction of healthy cells of the patient.
Several methods are used to check this, and devices such as the Elekta™ Synergy™ device employ two sources of radiation, a high energy accelerator capable of creating a therapeutic beam and a lower energy X-ray tube for producing a diagnostic beam. Both are mounted on the same rotateable gantry, separated by 90°. Each has an associated flat-panel detector, for portal images and diagnostic images respectively.
In our earlier application WO-A-99/40759, we described a novel coupling cell for a linear accelerator that allowed the energy of the beam produced to be varied more easily than had hitherto been possible. In our subsequent application WO-A-01/11928 we described how that structure could be used to produce very low energy beams, suitable for diagnostic use, in an accelerator that was also able to produce high-energy therapeutic beams. Later, in WO2006/097697A1 we described how to switch between those high- and low-energy beams at high speed. The disclosure of all of these prior disclosures is hereby incorporated by reference. The reader should note that this application develops the principles set out in those applications, which should therefore be read in conjunction with this application and whose disclosure should be taken to form part of the disclosure of this application.